This invention relates to the confinement of plasmas by magnetic fields and, more particularly, to an improved method and apparatus for the formation of a spheromak plasma (compact torus).
Devices employed for the containment of plasmas by magnetic fields may have various configurations. Two well-known types of such devices are the open-ended type, such as the magnetic mirror type, and the toroidal type, such as the tokamak. The underlying principle of all types of such containment devices is the containment of a hot, dense ionized gas away from physical walls for a time sufficient to allow fusion reactions to take place.
An advantage of the mirror-type device is that it has a coil-blanket topology which does not link the plasma. However, the mirror-type open ended apparatus has a disadvantage in that since the magnetic field lines do not close upon themselves, the trapped charge particles may escape while travelling along the magnetic field lines which define their spiral orbits. It occurred to many people in the early days of fusion research that mirror end losses could be easily eliminated simply by bringing the two ends of the straight cylinder on themselves, thus forming the well-known torus device.
The toroidal-type devices have an advantage in that plasma is well confined in the closed magnetic field lines. Since the ions tend to remain in a spiral orbit about a given set of magnetic field lines the continuity of the magnetic field lines inside the apparatus enhances containment. A tokamak clearly has this above-mentioned advantage but suffers from a difficult topology in which the coil blanket links the toroidal plasma.
The spheromak combines the most advantageous aspects of the above-discussed toroidal and mirror schemes. The spheromak is characterized by magnetic field lines which are closed, as in a tokamak, and by a coil blanket topology which does not link the plasma, as in a mirror-type device.
Among the advantages of this speromak formation scheme is the ability to keep the physical structure of the apparatus away from the plasma, thus reducing absorbed impurities and keeping the plasma "hot." Also, the spheroidal blanket simplifies the design and construction of the reactor apparatus. The magnetic field configuration of the spheromak includes both toroidal and poloidal components, but the toroidal component is maintained entirely by plasma currents, and, therefore, it vanishes outside the plasma. The outward pressure of the toroidal field and of the plasma is balanced by the inward pressure of a poloidal field.
For additional background discussions relating to the spheromak configuration, the reader is referred to S-1 Spheromak, Princeton University, Plasma Physics Laboratory, Aug. 24, 1979, the disclosure of which is hereby incorporated by reference.
Three known methods of spheromak plasma formation suitable for spheromak start-up have been experimentally confirmed. The first of these is the so-called "Marshall gun" approach, which is discussed in Alfven, Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy 31 (1958). This approach is characterized by the establishment of an initial poloidal field, followed by the application of toroidal flux through an electrode system. Plasma inertia is relied upon to immobilize the toroidal flux while the poloidal field lines are reconnected within the plasma. This approach has the disadvantage of requiring formation on a dynamic time scale, leading to questions of whether the internal poloidal flux is adequately reconnected. Also, since an electrode system is used, this formation scheme may suffer from problems of erosion and impurity influx, causing plasma cooling problems.
Another known scheme suitable for spheromak start-up is the familiar reversed-field theta-pinch approach, as discussed in Centre de Recherches en Physique des Plasmas, Lausanne, Switzerland (1978-79). This scheme is quite similar to the Marshall-gun approach, and thus suffers from the same disabilities. The major difference between the two approaches is that the geometry of the plasma forming structure is rotated by 90.degree. relative to that of the Marshall-gun approach, thus producing radial, rather than axial, plasma acceleration.
An improved method and apparatus for inductively forming a detached spheromak plasma configuration wherein the plasma may be contained at a substantial distance away from physical walls is disclosed in a published report entitled S-1 Spheromak, Princeton Plasma Physics Laboratory, Princeton, N.J. (Aug. 24, 1979). The original S-1 Spheromak described in that report is useful for forming a hot plasma, and for generating possibly large quantities of X-rays and neutrons, and can be used in numerous instances where neutrons are needed, as for example in the formation of medical isotopes.
The original S-1 Spheromak, which is illustrated in FIG. 1, includes a toroidally-shaped flux core having a radially interior major radius side and a radially exterior major radius side which includes both poloidal and toroidal magnetic field generating coils; a generally spheroidal vacuum vessel for enclosing the flux core; a pair of external equilibrium field coils for supporting the detached plasma; and a pair of pinching coils for pinching off or severing a portion of the plasma and for causing poloidal magnetic field line reconnection, such that the detached plasma may be contained at a distance from physical structure.
The original S-1 spheromak has been described in the aforementioned report to operate by energizing the external equilbrium field coils to produce a first poloidal magnetic field; energizing the poloidal coil of the flux core to produce a second poloidal magnetic field, thereby to produce a composite poloidal field which is stronger on the radially exterior major radius side of the flux core than on the radially interior major radius side; energizing the toroidal coil of the flux core to initiate a plasma discharge and to emit toroidal flux which becomes trapped in, and expands, the poloidal flux, such that the plasma expands toward the radially interior major radius side of the flux core, and; pinching off a portion of the distended plasma by energizing the pinching coils so as to produce a detached spheromak plasma. The time variation of the currents applied to each of these coils in accordance this prior method is illustrated in FIG. 2.
In U.S. Pat. No. 4,363,776 which is assigned to the same assignee as the present application, the original S-1 Spheromak is described, and its operation to produce a detached spheromak plasma with and without the use of the pinching coil is also described. In accordance with the operation of the original S-1 Spheromak without the use of the pinching coils described in U.S. Pat. No. 4,363,776, the poloidal coil is deenergized at a particular time to produce the spheromak plasma. That is, after the external and poloidal coils have been energized to form the poloidal field and the toroidal coil has been energized to initiate a plasma discharge and to emit toroidal flux which becomes trapped in and expands the poloidal flux, such that the plasma expands toward the major axis of the system, the poloidal coil current is turned off to produce a detached spheromak plasma. Thus, it is known to detach the spheromak plasma by either energizing pinching coils or turning off the poloidal coils at an appropriate time. Experimenters with these two approaches have, however, found that at most only approximately 50 percent of the plasma can be detached from the flux core in this manner.
Another limitation of the original S-1 Spheromak is that its arrangement of external equilibrium coils and poloidal field coils resulted in poloidal flux intercepting the flux core and attendant plasma loses to the flux core.